Flow cytometry method for illuminating a measurement sample including cells as a measuring object with laser beam and measuring the size and shape of each cell by using scattered light and fluorescence from the measurement sample, is disclosed in WO publication No. 2006/103920 for example. According to this flow cytometry method, a measurement sample including cells as a measuring object is surrounded by sheath liquid and is squeezed in a sheath flow cell to arrange the cells in one straight line to pass the flow cell. In this manner, a plurality of cells are suppressed from simultaneously passing through a detection region in the sheath flow cell.
However, cells may aggregate in the measurement sample. The existence of aggregating cells (a plurality of cells that aggregate) makes it difficult to accurately measure the sizes and shapes of the respective cells in the measurement sample for example.
In view of the above, according to an analysis apparatus disclosed in WO publication No. 2006/103920, forward-scattered light from a measurement sample illuminated with laser beam is detected. Then, a ratio between the difference integration value of the signal waveform of the obtained forward-scattered light and the peak value of the signal waveform is used to determine whether the signal waveform includes a trough or not to thereby distinguish between aggregating cells and non-aggregating cells (a plurality of cells that do not aggregate and each of which exists as a single cell).